[Torch Elastic][Draft] Refactor SubprocessHandler to separate module for easier subclass (#120373)
Summary:
## No Functional Change
- Refactor Subprocess Handler into a separate folder for easier subclassing
- SubprocessHandler
- added `local_rank_id` in `SubprocessHandler` to make it available as a field in the class
- pass in `local_rank_id` from subprocess start
Test Plan: No functional changes.
Differential Revision: D54038627
#suppress-api-compatibility-check
Pull Request resolved: https://github.com/pytorch/pytorch/pull/120373
Approved by: https://github.com/kurman
